1. Field of the Invention
The present invention relates to an interchangeable-lens type imaging apparatus and a lens unit mountable on the imaging apparatus.
2. Description of the Related Art
Some recent interchangeable-lens type cameras, represented by single-lens reflex cameras, are equipped with not only a still image capturing function but also a moving image capturing function. This function enables various moving image expressions by interchanging lenses, which have not been achieved by a conventional lens-integrated type video camera.
Exposure in the imaging apparatus is generally adjusted by combining adjustments of a diaphragm and a shutter speed. However, when the exposure is adjusted by increasing the shutter speed in the case of high object luminance, flickering of an image becomes conspicuous during moving image capturing, thus causing a reduction of moving image quality. Particularly, in the interchangeable-lens type camera, which is characterized by an image expression where the diaphragm is opened to decrease a depth of field and to blur a background, the shutter speed is required to be increased more when the diaphragm is opened, consequently making an image quality reduction conspicuous.
Under such a shooting condition, an effective way to carry out photographing is to attach a neutral density (ND) filter to a lens to decrease a light amount by the filter, thus preventing an increase of the shutter speed. However, it is bothersome to carry the ND filter separately from the imaging apparatus and attach the filter according to a shooting condition in each case. It is, therefore, convenient that a detachable ND filer is built in an optical path of a camera body.
Conventionally, lens-integrated type video cameras generally have an ND filter built-in. However, when an optical member such as an ND Filter is inserted into or retracted from a photographic optical system, a focus position of the photographic optical system shifts due to a difference in refractive index between optical members or a thickness of the member. Thus, Japanese Patent Application Laid-Open No. 2003-241078 discusses a lens-integrated type video camera that detects an insertion state of an ND filter and corrects defocusing by driving a focus control lens (focus lens) according to the insertion state.
In the case of lens-integrated type video cameras, such as that discussed in Japanese Patent Application Laid-Open No. 2003-241078, a thickness of the filter and optical characteristics of the lens are known beforehand. Thus, defocusing can accordingly be corrected by storing a correction amount of the focus lens position according to the insertion state of the ND filter in a camera body beforehand.
However, in the case of interchangeable-lens type cameras, since various types of lens units different in optical characteristics may be attached to a camera body, it is difficult to store the correction amount of the focus lens position in the camera body beforehand. Similarly, a lens unit may be attached to various types of camera bodies having different filters, and, hence, it is difficult to store the correction amount in the lens unit beforehand. Consequently, in the interchangeable-lens type cameras, defocusing caused by insertion or retraction of the ND filter cannot be corrected, unlike the case of lens-integrated type video cameras. This issue becomes conspicuous when a moving image is captured with a general rear focus type lens mounted as a lens configuration of a zoom lens of the video camera, as described below.
In the case of the rear focus type lens, an in-focus state during zooming is maintained by controlling a magnification variation lens (zoom lens) and a focus lens in association with each other so as to satisfy a predetermined positional relationship (an electronic cam locus) based on an object distance. However, when the ND filer is inserted to cause a change in the focus position, the positional relationship between the zoom lens and the focus lens defined by the electronic cam locus shifts, thus disabling maintenance of the in-focus state during zooming. In the case of capturing a moving image, an image captured during zooming is also recorded. As a result, unless a change in the focus position caused by the insertion of the ND filter can be corrected, an out-of-focus image may be recorded during zooming.